Several examples of heads of this type are currently available on the market. These heads comprise a casing internally of which a winding-reel is housed for winding a wire cutting spool. Two portions of the cutting wire protrude outwardly through dedicated openings in the casing. The winding-reel is associated to the casing and can take on an engagement position, wherein it is rotatingly solid with the casing, and a free position, wherein it can rotate with respect to the casing. Notoriously, the free position of the winding-reel is exploited to cause unwinding of the wire spool with the purpose of restoring the length of the cutting wire portions which protrude outward of the casing. The automatic cutting wire unwinding device allows the winding-reel to move between the engagement position and the free position without being necessary to stop the trimmer head and perform manual operations.
The activation of the automatic unwinding device exploits the rotational speed variation of the head resulting from the shortening, due to wear, of the wire portions which protrude outward of the casing. The shortening of such portions reduces the moment of inertia of the head, which consequently increases its rotational speed. The automatic cutting wire unwinding device is activated as a result of the increased centrifugal force acting on one or more masses suitably placed inside the head. The activation of the unwinding device causes a lengthening of the protruding wire portions and an increase of the moment of inertia of the head. Hence, the head slows down, thereby reducing the force of inertia acting on the masses which cease to activate the unwinding device.
The heads currently available therefore provide use of a winding-reel whereon the cutting wire is wound. Once the cutting wire is used up, a new cutting wire needs to be wound onto the reel in order that the terminated spool can be restored. This operation, which is carried out without the head having to be disassembled, is rather long in that a manual rotation of the winding-reel is required until complete winding of the fresh spool thereon.
An example of a head currently available is described in WO2013/179318.